A. Technical Field
The present invention relates to a microelectromechanical structure, and more particularly, to systems, devices and methods of compensating effect of thermo-mechanical stress that causes offset and sensitivity drift in a corresponding sensing output.
B. Background of the Invention
A microelectromechanical structure (MEMS) is widely applied as a sensor to measure acceleration, rotation, pressure and many other physical parameters. The MEMS device is normally formed on a silicon substrate using a micromachining process, and thus, adopts characteristic feature sizes of several micrometers. Such miniaturized devices transduce mechanical movement to electrical signals that may indicate the level of the interested parameters. Examples of the MEMS device include accelerometers, gyroscopes, magnetometers, and pressure sensors. Various MEMS devices have been widely employed in applications ranging from common consumer products to specialized products used under extreme environments, and nowadays, they may be easily found in automotive parts, mobile phones, gaming devices, medical appliance, and military applications.
Many MEMS devices rely on capacitive sensing between a moveable electrode and a stationary electrode, and one example of such MEMS devices is a micro-machined accelerometer. The accelerometer comprises a proof mass that is suspended above a silicon substrate, and responds to acceleration with respect to a certain sensing axis. The moveable electrode and the stationary electrode are coupled on the proof mass and the stationary device substrate, respectively. Upon acceleration, the moveable electrode experiences a relative location change with respect to the stationary electrode, resulting in a capacitive change of the sensing capacitor formed between these two electrodes. To be specific, the capacitive change may be induced by variation of the capacitive gap or area of the sensing capacitor that is associated with the relative location change between the electrodes. In many prior art devices, this sensing capacitor is monitored with respect to another reference capacitor whose capacitance is maintained at a constant value regardless of the acceleration rate.
Thermo-mechanical stress may introduce an intrinsic mismatch between the sensing and reference capacitors, and ultimately, lead to offset or sensitivity drift to a sensing output even though no acceleration is applied to induce any capacitive change yet. In an ideal situation, the capacitive variation of the sensing capacitor should only be associated with the acceleration, and does not exist when no acceleration is involved. However, thermo-mechanical stress may be accumulated in the MEMS device during the course of manufacturing, soldering, packaging and device aging. Non-uniform stress builds up within the substrate and the device structure including the suspended proof mass, and unavoidably causes the substrate to warp and the proof mass to shift or tilt. In rare cases, the thermal stress impacts the sensing and reference capacitors equally, such that their gap and area variations might happen to cancel out and result in no capacitance mismatch between the capacitors. In most cases, the thermal stress impacts the sensing and reference capacitors differently. Various structures including the moveable and stationary electrodes are subject to different displacements. The sensing output from a sensor interface circuit may reflect such displacements resulting from the non-uniform thermal stress, and lead to an offset value and a sensitivity drift for the sensed acceleration.
Apparently, device performance of a capacitive accelerometer is compromised due to the thermo-mechanical stress. Such performance degradation is commonly shared by the MEMS devices that primarily rely on suspended proof masses and capacitive electrodes for transducing and sensing mechanical movement. There is a need to compensate the thermo-mechanical stress that builds up during the course of manufacturing, packaging, assembly and regular operation.